Sulfur is an inexpensive, ubiquitous material that can be mined but is more commonly derived as a by-product from flue gas desulfurization processes and the purification of petroleum. In the 1970's, the U.S. government predicted that sulfur produced as a by-product would increase to such an extent that sulfur production would exceed demand and create storage and economic problems. Therefore, the U.S. Department of the Interior initiated a program in 1972 to develop alternative ways of utilizing sulfur.
Sulfur is thermoplastic in nature, therefore it can be melted and then cooled back to solid form. Because of this property, it can be mixed together with aggregate or fillers to form sulfur-based concretes and composite materials that can be used as an alternative to conventional hydraulic cement concretes. Sulfur concretes have very low permeability, high strength, and are resistant to many harsh chemicals (e.g., strong acids) that degrade conventional hydraulic cement concretes. In particular, after five years of industrial testing in over 50 corrosive environments, sulfur concrete materials exhibit excellent mechanical properties when compared to Portland cement concretes. See United States Department of the Interior Bureau of Mines Report of Investigations/1988, Wrzesinski, et al. “Permeability and Corrosion Resistance of Reinforced Sulfur Concrete” page 2, first column, middle of second paragraph; Mc. Bee W. C., Sullivan T. A. Development of specialized sulfur concretes, U.S. Department of the Interior, 1979, Bureau of Mines Report No. 8346, p. 22; Vroom A. H. Sulfurcrete Another option in the energy /Materials picture//Military Engineering.-1979. 71.-N 462, p. 250-252; and Sulfur concrete—golden opportunity//Consr. Prod.-1984. 27.-N1, p. 38.
However, pure sulfur goes through an allotropic solid phase transition, upon cooling below 95.5° C., from the monoclinic to the orthorhombic form which is more dense and occupies less volume. In other words, cooling of the sulfur results in an increase in density (shrinkage of the matrix) which introduces physical instabilities in the solid and makes the material highly stressed and susceptible to cracking and mechanical failure.
To remedy the problem caused by the allotropic solid phase transition of sulfur, scientists developed modified sulfur concretes. One of the modified sulfur concrete cements developed under the U.S. Department of Interior's program contains dicyclopentadiene (DCPD), and oligomers of cyclopentadiene, primarily the trimer through the pentamer. This cement allows the sulfur to polymerize such that the solid phase transition on cooling is suppressed and the resulting product is very durable.
A major drawback of the DCPD-modified cement, is that the cost of DCPD modifiers is relatively high and they are not readily available worldwide. Furthermore, DCPD imparts an unpleasant odor to the sulfur cement and its vapor is toxic even at low concentrations. See Kinkead, et al. “The Mammalian Toxicity of Dicyclopentadiene” Toxicology and Applied Pharmacology, 20 552-561 (1971) and Gregor R., Hackl A., A New Approach to Sulphur Concretes. Ch. In Advances in Chemistry Series, N 165, American Chemical Society, Washington, 1978, pp. 54-78.
Thus, the development of new readily available, less expensive modifiers is needed to expand the potential applications for low-cost sulfur concrete products. Furthermore, currently used methods of polymerization do not ensure homogenization within the sulfur matrix, which can impact durability and strength properties of the sulfur concrete or composite products.